1. Field of the Invention
The present invention relates to a surface mounted (SMD) piezoelectric vibrator where a piezoelectric vibrating reed is sealed in a cavity formed between two bonded substrates, a method of manufacturing the piezoelectric vibrator, and an oscillator, an electronic device, and a radio clock that each include the piezoelectric vibrator.
2. Description of the Related Art
In recent years, a piezoelectric vibrator, which employs crystal or the like as a time source, a timing source of a control signal or the like, a reference signal source or the like, has been used in a cell phone or a portable information terminal device. Various piezoelectric vibrators have been known as this kind of piezoelectric vibrator, but a surface mounted piezoelectric vibrator has been known as one of the piezoelectric vibrators. As this kind of piezoelectric vibrator, there is generally known a piezoelectric vibrator having a three-layer structure where a piezoelectric substrate including a piezoelectric vibrating reed is bonded to a base substrate and a lid substrate so as to be interposed between the base substrate and the lid substrate in a vertical direction. In this case, the piezoelectric vibrator is received in a cavity (closed chamber) that is formed between the base substrate and the lid substrate. Further, in recent years, there has been developed a piezoelectric vibrator having not a three-layer structure but a two-layer structure.
This type of piezoelectric vibrator has a two-layer structure where a base substrate and a lid substrate are directly bonded to each other and a piezoelectric vibrating reed is received in a cavity formed between both the substrates. The two-layer structure type piezoelectric vibrators are superior to three-layer structure type piezoelectric vibrators in terms of the reduction in thickness. Accordingly, the two-layer structure type piezoelectric vibrators are preferably used. As one of the two-layer structure type piezoelectric vibrators, there is known a piezoelectric vibrator where a piezoelectric vibrating reed and external electrodes formed on a base substrate are electrically connected to each other by using conductive members that are formed to pass through the base substrate (see Patent Citations 1 and 2).
As shown in FIGS. 22 and 23, a piezoelectric vibrator 200 includes a base substrate 201 and a lid substrate 202 that are anodically bonded to each other with a bonding film 207 interposed therebetween, and a piezoelectric vibrating reed 203 that is sealed in a cavity C formed between both the substrates 201 and 202. The piezoelectric vibrating reed 203 is, for example, a tuning-fork type vibrating reed, and is mounted on the upper surface of the base substrate 201 in the cavity C by a conductive adhesive E.
Each of the base substrate 201 and the lid substrate 202 is an insulating substrate that is made of, for example, ceramic or glass. Through holes 204, which pass through the substrate 201, are formed at the base substrate 201 of both the substrates 201 and 202. Further, a conductive member 205 is fitted to the through hole 204 so as to close the through hole 204. The conductive member 205 is electrically connected to an external electrode 206 formed on the lower surface of the base substrate 201, and is electrically connected to a piezoelectric vibrating reed 203 mounted in the cavity C.
Patent Citation 1: JP-A-2002-124845
Patent Citation 2: JP-A-2006-279872
Meanwhile, in the above-mentioned two-layer structure type piezoelectric vibrator, the conductive member 205 has two functions, that is, a function to close the through hole 204 so as to maintain airtightness in the cavity C and a function to connect electrically the piezoelectric vibrating reed 203 to the external electrode 206. In particular, if the conductive member insufficiently comes into close contact with the through hole 204, there is a concern that the airtightness in the cavity C is impaired. Further, if the conductive member insufficiently comes into close contact with the conductive adhesive E or the external electrode 206, the malfunction of the piezoelectric vibrating reed 203 is caused. Accordingly, even to eliminate these troubles in the related art, it is necessary to form the conductive member 205 so that the conductive member completely closes the through hole 204 while being solidly fixed to the inner surface of the through hole 204 and recesses are not formed on the surfaces of the conductive member.
However, Patent Citations 1 and 2 disclose that the conductive member 205 is formed by conductive paste (Ag paste, Au—Sn paste, or the like), but do not disclose a specific method of manufacturing the conductive member, that is, how to actually form the conductive member.
When conductive paste is used, it is generally necessary to harden the conductive paste by firing the conductive paste. That is, it is necessary to harden the conductive paste by performing firing after the through hole 204 is filled with the conductive paste. Meanwhile, if firing is performed, organic materials contained in the conductive paste are evaporated and disappear. For this reason, the volume of the fired conductive paste is generally decreased in comparison with the volume of the unfired conductive paste (for example, when Ag paste is used as the conductive paste, the volume of the conductive paste is decreased by about 20%). For this reason, even though the conductive member 205 is formed using the conductive paste, there is a concern that recesses are formed on the surfaces of the conductive paste or a through hole is formed at the center of the conductive paste in an extreme case.
As a result, there has been a possibility that the airtightness in the cavity C is impaired or electric conductivity between the piezoelectric vibrating reed 203 and the external electrode 206 is impaired.